In present day design and production systems, it is often necessary to measure the concentration of a material in a sample. For example, it is often necessary to measure concentration of particulate solid material in a sheet sample. In the tobacco products industry, in particular, it is important to measure concentration and consistency of carbon in a carbon-containing sheet. Such carbon-containing sheets typically comprise a mixture of tobacco and carbon, as described, for example in U.S. patent application Ser. No. 07/408,433, filed Sept. 15, 1989, entitled Smoking Article With Improved Means For Delivering Flavorants, which is assigned to the Assignee of the present invention. Crimped plugs of the carbon-containing sheet may be used as a flavor carrier by, for example, applying menthol and glycerine to the sheet. These carbon-containing sheets may also be used as filters for selectively absorbing particular compounds. In order to ensure high quality and uniformity of cigarettes it is desirable to measure the carbon concentration in the carbon-containing sheets to ensure that production tolerances are met.
Heretofore, a carbon tetrachloride (CCl.sub.4) vapor absorption technique has been employed to measure carbon concentration in a carbon-containing sheet. In this technique, the sheet is exposed to CCl.sub.4 vapors, and the CCl.sub.4 absorption rate is monitored to monitor carbon concentration. Alternatively, a "wet digestion" technique has also been employed to measure carbon concentration. In a wet digestion process, a sample of the sheet is repeatedly soaked in solutions of HCl, HCl+(NH.sub.4)(C.sub.2 O.sub.2), NaOH and Cu(NH.sub.3).sub.4 (OH).sub.2 to dissolve the tobacco, leaving the carbon behind as residue. The residual carbon may be weighed to obtain a carbon concentration. Other chemical processes for measuring carbon concentration are also known.
Unfortunately, vapor absorption, wet digestion or other chemical processes are time consuming, and cannot be employed in an "on line" monitoring system for performing quality control measurements on carbon-containing sheets during production. Moreover, wet digestion or other chemical techniques are "destructive" measuring techniques, in which at least a portion of the sheet needs to be destroyed in order to obtain the measurement. Nondestructive measuring techniques are preferred, to allow carbon concentration and distribution to be measured without destroying the sheet.
Photometric techniques have been employed in the measuring arts. For example, U.S. Pat. No. 4,252,443 to Lucas et al. discloses a blackening sensor which analyses a signal generated by the detection of scattered light reflected from a succession of small illuminated areas on the surface of a sheet, to provide an indication of the degree of blackening of the sheet. However, this system employs many analog circuits, such as filters, clippers, averaging circuits and thresholding circuits which are complicated, inaccurate and require frequent adjustment.
Another photometric technique is described in U.S. Pat. No. 4,737,464 to McConnell et al., which describes a solid state optical assay imaging apparatus including a 256 .times.255 optical memory to store a digital image of a light modulated scanned object. A special optical memory is employed, having memory cells which decay due to the modulated light over a period of time as a function of the initial intensity of the light. The amount of time for 50% of the cells to change state is determined. This "half life" measurement time is then employed to obtain a measurement of optical concentration. Although this system uses digital circuits, the optical memory is expensive, and the need to rely on "half life" decay may create an inaccurate measurement.